Does Flight or Fight need a valid target?

Yes, Fight or Flight requires a valid target to be played.

According to the rules of Riftbound and the associated FAQs:

• Targeting Requirement: Because the card text instructs you to "Move a unit from a battlefield to its base," it targets that unit. Rule 352.7 states: "In order to put a spell or ability on the chain, valid choices must be made for all targets."
• Declaration: You must declare the target for Fight or Flight at the moment you finalize it (put it on the chain), before your opponent has the opportunity to react (RiftJudge FAQ #6474).
• Validity: If there are no units at a battlefield to target, you cannot play the card, as you cannot satisfy the targeting requirement.

This is consistent with the general rule that any spell or ability mentioning a game object (like "a unit") targets that object unless it falls under a specific exception (such as affecting all units or being part of a cost/restriction), as outlined in Rule 352.10.